Recently, there has been growing interest in energy storage technologies. As the application fields of energy storage technologies have been extended to mobile phones, camcorders, lap-top computers and even electric cars, efforts have increasingly been made towards the research and development of electrochemical devices. In this aspect, electrochemical devices have attracted the most attention. The development of rechargeable secondary batteries has been the focus of particular interest.
Among currently available secondary batteries, lithium secondary batteries developed in the early 1990's have received a great deal of attention due to their advantages of higher operating voltages and higher energy densities than traditional batteries such as Ni-MH batteries and the like.
Generally, a lithium secondary battery is fabricated by making a cathode and an anode using a material capable of intercalating/deintercalating or alloying/dealloying lithium ions, and filling an organic electrolyte solution or a polymer electrolyte solution in between the cathode and the anode, and produces electrical energy by an oxidation/reduction reaction when the lithium ions intercalate and deintercalate on the cathode and the anode.
Currently, as an electrode active material (anode active material) for an anode of a lithium secondary battery, a carbon-based material is primarily being used. Among the carbon-based material, graphite has a theoretical capacity of about 372 mAh/g, and an actual realizable capacity of currently available graphite is from about 350 to about 360 mAh/g. However, a carbon-based material such as graphite is insufficient for a high capacity lithium secondary battery. To meet the demand, another anode active material is a metal such as silicon (Si) and tin (Sn) that exhibits a higher charge/discharge capacity than a carbon-based material and may be electrochemically alloyed with lithium and its oxide or alloy. However, a metal-based (non-carbon-based) anode active material experiences cracking and pulverization due to a large volume change involved in lithium charging/discharging, and as a consequence, a secondary battery using a metal-based anode active material has drawbacks of a drastic capacity drop and a short cycle life during charging/discharging cycles.
Meanwhile, due to its high capacity property, a cathode active material using nickel, manganese, or cobalt, particularly, a manganese-rich NMC-based or MNC-based cathode active material gain a great attention, but such a cathode active material has an excessive amount of Mn3+ ions present on the surface, and Mn3+ goes through a disproportionation reaction (2Mn3+->Mn4++Mn2+). Mn2+ ions generated during the disproportionation reaction are released into an electrolyte solution, resulting in a significant degradation in cycle and storage characteristics of the battery. To solve this, attempts have been made to diffuse lithium through a direct contact between a perforated current collector (for example, a foil) and a lithium metal (in the shape of a plate or foil), or to predope a lithium metal through a short circuit between electrode active materials.
However, when a perforated current collector is used, there are problems with a reduced loading amount of electrode active materials and consequently a reduced capacity as well as a reduced contact area of an electrode active material with a current collector decreases and consequently an increased resistance to an electric current. Also, in the case of a certain cathode active material for high capacity, the problem with collapse of a crystal structure based on a voltage range occurs, and resulting metal ions are well known to deteriorate a solid electrolyte interphase (SEI) layer created on the anode surface.
Accordingly, there is still the demand for a new prelithiation method to prevent reduction in capacity and cycle life of a battery as well as an irreversible capacity reduction occurring when a metal-based anode active material is used.